You have to lose a lot more than just one match to get demoted. According to your match history and LoLKing your stats are (mostly) fine, although your overreliance on Fiddlesticks mid/top lane might be problematic. I can understand wanting to abuse a single champion for a long stretch of time, as I've been doing that for a long time now with Nami, but just because Nami is my best doesn't mean I am unable to play other champions or other roles when there is a (rare) situation that she doesn't work. If I need someone tankier, Braum or Thresh are good choice, and if I need an initiator or first pick, Thresh is my best option.
Either you have poor teamplay, poor objective play, are toxic in some capacity, or any combination of the three. Given the manner in which you phrased your post and the fact that you posted to begin with, I think I'm glad I am far, far away from your elo. You're in the borderline between Silver and Gold, and this is a hard elo for a lot of people to get past. You can't win by stomping the early game anymore, it's going to become just as important to know how to team fight as it is to know how to succeed at the early game. Looking at the team comps, I see a pick comp going against a team where your only "tank" was a Lee Sin who built Last Whisper and Spirit of the Elder Lizard, and coming out on top. Of course you lost! You're playing in the mid lane and are ending up with the most gold or close to it on your teams, at least from the games I looked at. Are you able to turn that gold into a won team fight or an objective? Given you're playing Fiddlesticks, who isn't exactly known for his ability to auto-attack, I'm not surprised you're running into issues.